Method of and apparatus for establishing a reference point on a gradation correction curve

Abstract

A highlight point (HL R , HL G , HL B ) and/or a shadow point (SD R , SD G , SD B ) on a gradation correction curve are established in consideration of decoloration and color fog of an original. Density values of respective primary colors are obtained for each pixel on an original. Average density is computed for each pixel and a cumulative histogram of average density distribution is generated. Temporary reference density values corresponding to prescribed cumulative frequencies are found on the cumulative histogram. Reference color density values (D RH , D GH , D BH ) are computed by averaging the density of each color for the pixels having average density between the temporary reference density and terminal density on the cumulative histogram. The highlight and shadow points are specified with one of the reference color density values.

What is claimed is: 
     
       1. A method of establishing a reference point on a gradation correction curve employed in gradation correction of image data, said method comprising the steps of: (a) preparing image data representing density values of a plurality of basic colors with respect to each pixel in an original;   (b) averaging said density values of respective basic colors with respect to each pixel to obtain an average density D M  of each pixel;   (c) generating a histogram representing average density distribution in the form of a relation between said average density D M  and a number of pixels N p  ;   (d) generating a cumulative histogram representing said average density distribution in the form of a relation between said average density D M  and a cumulative number of pixels RN;   (e) finding a reference average density D Mrf  corresponding to a prescribed value of a reference cumulative number of pixels RN rf  on said cumulative histogram;   (f) selecting one of two terminal density values D Mn  at which said cumulative histogram saturates and which is closer to said reference average density;   (g) finding a set of pixels having said average density in a range essentially between said reference average density D Mrf  and a selected terminal density value D Mm  ;   (h) averaging said density values of said plurality of basic colors for said set of pixels with respect to each of said plurality of basic colors, to thereby obtain reference color density values D rf  for respective basic colors; and   (i) establishing said reference point on the basis of said reference color density values D rf .   
     
     
       2. A method in accordance with claim 1, wherein: said gradation correction curve is defined on a coordinate plane whose first coordinate represents a first density DI before gradation correction and whose second coordinate represents a second density DO after gradation correction; and   said gradation correction curve is drawn through said reference point specified with one of said reference color density values D rf  as said first coordinate and a prescribed value DO rf  as said second coordinate.   
     
     
       3. A method in accordance with claim 2, wherein said gradation correction curve is generated for each of said plurality of basic colors.   
     
     
       4. A method in accordance with claim 3, wherein said plurality of basic colors essentially consist of primary colors. 
     
     
       5. A method in accordance with claim 4, wherein: said reference point is either of a highlight point or a shadow point; and   at least one of said highlight point and said shadow point is established by said steps (a) through (i).   
     
     
       6. A method in accordance with claim 5, wherein said reference point for each of said plurality of basic colors is specified with one of said reference color density values D rf  for a corresponding color as said first coordinate. 
     
     
       7. A method in accordance with claim 5, wherein said reference point for each of said plurality of basic colors is specified with the largest value among said reference color density values D rf  as said first coordinate. 
     
     
       8. A method in accordance with claim 5, wherein said reference point for each of said plurality of basic colors is specified with the smallest value among said reference color density values D rf  as said first coordinate. 
     
     
       9. An apparatus for establishing a reference point on a gradation correction curve employed in gradation correction of image data, said apparatus comprising: (a) means for preparing image data representing density values of a plurality of basic colors with respect to each pixel in an original;   (b) means for averaging said density values of respective basic colors with respect to each pixel to obtain an average density D M  of each pixel;   (c) means for generating a histogram representing average density distribution in the form of a relation between said average density D M  and a number of pixels Np;   (d) means for generating a cumulative histogram representing said average density in the form of a relation between said average density D M  and a cumulative number of pixels RN;   (e) means for finding a reference average density D Mrf  corresponding to a prescribed value of a reference cumulative number of pixels RN rf  on said cumulative histogram;   (f) means for selecting one of two terminal density values D Mm  at which said cumulative histogram saturates and which is closer to said reference average density;   (g) means for finding a set of pixels having said average density in a range essentially between said reference average density D Mrf  and a selected terminal density value D Mm  ;   (h) means for averaging said density values of said plurality of basic colors for said set of pixels with respect to each of said plurality of basic colors to thereby obtain reference color density values D rf  for respective basic colors; and   (i) means for establishing said reference point on the basis of said reference color density values D rf .   
     
     
       10. An apparatus in accordance with claim 9, wherein: said gradation correction curve is defined on a coordinate plane whose first coordinate represents a first density DI before gradation correction and whose second coordinate represents a second density DO after gradation correction; and   said gradation correction curve is drawn through said reference point specified with one of said reference color density values D rf  as said first coordinate and a prescribed value DO rf  as said second coordinate.   
     
     
       11. An apparatus in accordance with claim wherein said gradation correction curve is generated for each of said plurality of basic colors. 
     
     
       12. An apparatus in accordance with claim 11, wherein said plurality of basic colors essentially consist of primary colors. 
     
     
       13. An apparatus in accordance with claim 12, wherein said reference point for each of said plurality of basic colors is specified with one of said reference color density values D rf  for a corresponding color as said first coordinate. 
     
     
       14. An apparatus in accordance with claim 12, wherein said reference point for each of said plurality of basic colors is specified with the largest value among said reference color density values D rf  as said first coordinate. 
     
     
       15. An apparatus in accordance with claim 12, wherein said reference point of each of said plurality of basic colors is specified with the smallest value among said reference color density values D rf  as said first coordinate.